This invention relates to resistive microwave attenuators that are used for high peak and average power dissipation. More particularly, the invention in a preferred form relates to an attenuator element having an elongate cylindrical base with a plurality of electrically conductive, resistive, and non-conductive sections disposed on its surface to effectively form two L-pads having shunt resistances with a common ground, and where the series resistances are connected through a conducting strip having a predetermined impedance.
VHF T-pad attenuators are well known and consist basically of a pair of coupled series resistors and a grounded shunt resistor. Early designs recognize the importance of designing a high frequency attenuator to avoid any mismatch in impedance between a transmission line and the attenuator, resulting in a "reflection" back into the transmission line of a portion of the energy reaching the attenuator. Early designs, such as are described in Johnson et al., U.S. Pat. No. 2,620,396, used rod resistors fabricated from an insulating cylindrical substrate with a resistive film deposited on the surface. Highly conductive metal contacts were placed at both ends of the resistive film to provide connection to the resistor. These cylindrical resistors were easy to connect to the cylindrical coaxial center conductor of the terminals at both ends of the unit. The shunt resistor connected to ground between the two series resistors was originally formed on a round ceramic or glass disc substrate with a resistance film deposited on the surface of the disc. Highly conductive annular metal connectors at the outside circumference of the disc, and around a hole in the center of the disc, provided ohmic connections to the shunt resistor. The disc resistor was in the center of the attenuator, and its outer connection made ohmic contact to the inside diameter of the tubular body. The input and output resistors and shunt resistor were interconnected mechanically and electrically at the center hole of the shunt disc resistor.
Tapered outer conductors around the series rod resistors provided a better high frequency response and input impedance match. This taper around the cylindrical resistor is the most effective when it is an exponential, or logarithmic, taper so that the resistive parts of an attenuator are pure resistive elements at all frequencies. Exponential tapers around cylindrical film resistors are well known, and are described in U.S. Pat. Nos. 2,273,547, 2,399,645, and 2,438,915.
Maintenance of a constant resistance of the shunt disc resistor at all frequencies by providing an electrically correct ground plane has been difficult to accomplish. Design of the shunt resistor is important so that the attenuator will have a constant attenuation at all frequencies. Complex shapes for this element have been tried, but have not seemed to enjoy commercial success principally because of the difficulty of manufacture. An example of such an element is disclosed in U.S. Pat. No. 2,968,774. Various other arrangements of shunt resistors include a four-spoke arrangement as shown in U.S. Pat. Nos. 2,434,560 and 2,994,049, and a two-spoke design as shown in U.S. Pat. Nos. 3,739,305 and 3,996,534. These shunt resistors work effectively without an exponential housing only because the resistive element lengths are very short relative to a wavelength.
An advance in the state of the art of attenuator technology involved the use of microstrip construction with the resistors being placed on thin flat ceramic substrates. Examples of such construction are shown in U.S. Pat. Nos. 3,157,846, and 3,227,975 and 4,309,677. These assemblies were facilitated by developments in hybrid semiconductor manufacturing methods which used ceramic substrates. However, for good high frequency performance, flat resistors laid out on a ceramic surface are difficult to house to provide proper impedance. This is because of the uniform spacing to the ground plane on the opposite side of the ceramic substrate which results in an inability to vary the impedance around the resistors along their length. To a certain extent, this problem has been solved by reducing the physical size of the resistive assembly to the point where the lack of the proper tapered housing for the resistances provides relatively minimal reactances at high frequencies. However, as the size of the resistive elements is reduced, the power handling capability of the unit is also reduced because of operating temperature limitations of the resistance film. In an effort to overcome this problem, high thermal conductivity ceramic substrates have been used to transfer and dissipate heat generated to an adjacent metal body.
At the present time, the T-pad design described in Barth, U.S. Pat. No. 3,665,347, has the best high frequency response relative to the size of the elements of any commercial unit. This device uses three cylindrical film resistors with exponential tapered housings around each resistor to provide pure resistance values at all frequencies. The shunt resistor in this design in mounted physically in a member perpendicular to the series resistors, thus requiring the exponential housing around the shunt resistor to also be maintained perpendicular to the axis of the body of the housing. Accordingly, while the unit is technically excellent, its manufacture is complicated and more costly than other designs. Furthermore, the creation of a secure mechanical and electrical connection of the series resistors with the shunt resistor can be problematic.
The invention provides an improvement on the design set forth in my earlier U.S. Pat. No. 3,665,347 in that it is much simpler to manufacture, but retains substantially the excellent performance characteristics of that unit. In a broad sense, rather than being a conventional T-pad attenuator, the attenuator of the invention comprises a pair of L-pads having shunt resistances with a common ground where the series resistances are connected through a conductive connecting strip having an impedance that matches the impedance of the adjacent L-pads. The impedance of the strip is selectively calculated for each attenuator size, and is determined by its physical characteristics such as material of construction, thickness, and width. In a specific embodiment, the invention comprises the afore described electrical configuration laid out on the peripheral external surface of a cylindrical dielectric base member. The various other resistive, conductive elements, and non-conductive sections are laid out in a predesigned orientation around the surface of the cylindrical base, with the series resistors generally consisting of film sleeves located at opposite ends of the cylinder. The two commonly grounded shunt resistors and the interconnecting conductive strips are generally located in a central portion of the cylindrical element. These elements are easily mounted in a conventional, exponentially tapered conductive metal housing similar to the one shown in U.S. Pat. No. 3,665,347, except that the perpendicular lug portion shown in that patent is no longer necessary because of the improved design of the elongated resistive element.
Accordingly, it is an object of the invention to provide a new attenuator design that is relatively easy to manufacture, but is able to handle high power with exceptional fidelity. It is yet another object of the invention to provide a coaxial attenuating element mounted inside of a tubular housing which acts as a ground for the shunt resistor, thus permitting encapsulation while maintaining good high frequency characteristics. It is yet another object of the invention to provide increased power handling capability of the resistive elements of a T-pad by providing a novel shunt resistor mounted on the same cylindrical substrate as the series resistors. By producing long resistors with a shape having a uniform current density, and mounting them in a housing that allows a constant resistance at all frequencies, high energies can be dissipated without damage to the resistive elements. These and other objects of the invention will be apparent from the following detailed description of a preferred embodiment thereof.